Underfeed stoker and its operation



Patented June 19,. 1945 2,378,805 UNDERFEED scroxnn Ann rrs orsaa'rrou Theodore S. Spicer and Calvert C. Wright, State College, Pa... assignors to Pennsylvania Bituminous Coal Research, Inc., Altoona, Pa., a corporation of Pennsylvania Application April 15, 1941, Serial No. 388,626

Claims.

This invention relates to underfeed stokers. especially of the type used for domestic, or home, heating.

Such underfeed stokers as constructed and operated heretofore with strongly coking coals are unsatisfactory because of the formation of what are called coke trees. That is, in the operation of stokers of this type a. column of coke may, and

advantages attendant thereon, can be repressed or prevented by subjecting the coal to oxidation prior to the time when it reaches the active combustion. zone. More particularly, in the practice of this invention coal as it passes to the retort of an underfeed stoker is agitated and contacted with an oxidizing agent to effect oxidation of the commonly does, build up above the retort. Coke trees .are objectionable because they interfere with eflicient combustion, slow down the response of the furnace to thermostatic demand for heat and result in over-shooting of the thermostat,

and make it "difficult to maintain low hold-fire,

rates without danger of the fire going out. Actually, coke tree formation may be so extensive and severe as actually to put the fire out despite operation of the stoker blower. Coke tree formation is particularly serious in the case of the strongly coking'or caking coals, whiclrprevents their use for these purposes and restricts the supply of satisfactory coal to certain sources.

It is among the objects of this invention to provide a method of operating stokers of the type referred to so as to avoid coke tree formation.

Another object is to operate underfeed domestic stokers in such manner as to permit the burning of strongly coking coals while repressing or eliminating the troubles heretoforeencountered in their use in such stokers.

Another object is to eliminate smoke in burning coking coals.

tion, is applicable to the various types of domestic tort portion of an .imderfeed stoker; and Fig. 2

is a tertical section taken on line 11-11, Fig. 1.

The invention is predicated upon our discovery that in the operation of an underfeed domestic stoker the formation ofjcoke trees, with the discoal prior to the time when heat conducted downwardly from the retort causes it to assume the plastic state. We have demonstrated that by such preoxidation of the coal its tendency, even in the case of strongly coking or caking coals, to

coke tree formation may be eliminated or at least reduced to the point where it is of no particular consequence.

Air constitutes the preferred oxidizing agent used in the practice of the invention, but it will be understood that other oxidizing agents may be used such, for example, as water, water vapor or .steam; In fact, there may be advantage in using a plurality of oxidizing agents, particularly air carrying water vapor.

The oxidation necessary for the purposes of the invention should be completed prior to the time when coking of the coal begins, 1. e., prior to the time when the coal becomes hot enough to develop plastic properties. For this reason the oxidizing agent is introduced into the coal at a point sumciently below the plastic zone of the retort to efiectthe desired oxidation.

The exact point at which: the oxidizing agent is introduced will depend upon the character of the coal, the amount of oxidizing agent used,' and the temperature conditions prevailing during the travel of the coal from the point of introduction to the plastic zone. For most purposes we now prefer to introduce the oxidizing agent at or toward the bottom of the retort, it having been found by actual operation in that manner that the coal is sufliciently oxidized, without overoxidation. of course, the air might be introduced at an earlier point in the coal travel, for instance, into the horizontal screw conveyor duct.

Likewise, the amount of oxidizing agent necessary for the purposes of the invention will depend upon'the same factors and the point of introduction of that agent. Using air as the oxidizing agent and introducing it into the retort of the type generally indicated in the drawing, we have found that with strongly coking coals from 15 to 2d per .cent of the total airsupplied to the retort will effect adequate oxidation to prevent coke tree formation, while in the case of more strongly coking coals as much as 35 per cent of the totalair is necessary to effect preoxidation adequate to prevent coke tree formation.

amount of air sufficient for the necessary oxidation, and to supply the balance of the air necessary for combustion to tuyeres situated in the active combustion zone. One reason for this is that it requires little change of existing underfeed stoker constructions. Another reason is that the oxidation should not be extensive enough to destroy entirely the coking qualities of the coal, and by supplying only enough air for preoxidation, or not a great excess thereover, avoidance of that result is accomplished most easily. Still another reason is to prevent a retreating action of the live fuel bed and also to afiord better holdfire control. To summarize, the amount of oxidizing agent necessary will depend upon the agent or agents used, the particular coal, the temperature in the treating zone, and the duration of exposure. An empirical mode of determining oxidation to the proper extent is to supply air in an amount such that the coal cokes to approximately egg size, which indicates that the coal has been oxidized sufiiciently to avoid coke tree formation but with retention of coking qualities desidable in the operation of stokers of this type.

Inasmuch as no appreciable amount of oxidation will occur when the coal is at room temperature, the preoxidation necessary for the purposes of the inventiontakes place only in the region in which the temperature of the coal increases as it moves upwardly through the retort into the plastic zone. To assist in effecting the oxidation throughout the mass of coal and thus to insure absence of coke free formation, the coal should be agitated in the oxidation zone. This may. be accomplished in various ways, as .will be understood in the art, but preferably we accomplish this by feeding the coal upwardly into the retort'by means of a vertically positioned screw conveyor.

By preoxidation in the manner described the formation of coke trees can be eliminated or at least reduced to the point where they do not cause difflculties in operation of a stoker. The combustion is improved and rendered more efficient. We find thatoperation of an underfeed stoker in tlle manner described not only improves combustion by elimination of the blanketing efiect of the coke tree, but also-increased efficiency by mor complete combustion, as indicated by the carbon dioxide content of the stack gases. Thus, in a stoker constructed and operated in the manner customary prior to our invention, without preoxidation of the strongly coking coal, the carbon dioxide content of the stack gases may be very low, for instance less than about 4 per cent, although visual examination sometimes would indicate that the stoker was operating efficiently. By the practice of our invention coke free formation is not only prevented 'but also the carbon dioxide content of the stack gases may be in the vicinity of 12 to 14 per cent. These factors combine to insure more eflicient combustion, more rapid responsewhen a thermostat calls for heat, avoidance of overheating such as has followed coke tree formation, and the screened nut coal in stokers operated without preoxidation.

It is recognized that in the stokers proposed or used heretofore some amount of air has in some instances been introduced into the coal as it passes to the retort, for instance through a smoke back. Air as thus introduced in prior stokers does not accomplish the results of our invention, however, either because the amount of air is insuiiicient to efiect any material amount of preoxidation, or because it is introduced into the plastic zone where it does not effect the necessary preoxidation, or for related reasons, and this is evidenced by the fact that in no underfeed domestic stoker or stoker operation known to us prior to our invention has coke tree formation been avoided. Also, the air has not been properly introduced heretofore'so that high induction' velocities and surging are avoided. Moreover, the air must not only be introduced at the proper place and in the proper manner, but be used in connection with a suitable agitator to effect intimate contact beween the coal and air.

Having reference now to the drawing, there is shown a retort I of conventional form provided with tuyeresZ and 3 forming the active combustion zone in the manner customary in the art. The retort is surrounded by a wind box 4 supplied with air from a main air duct 5. Coal is introduced to the retort by a horizontal screw 6 operating in a duct 1. The blower 5a which supplies air to mainduct 5 is shown as operated by a motor '50. The screw conveyor 6 is operated in any suitable manner as by motor 5|! ability to maintain low hold-fire rates. Additionor by a separate motor; such drive means form no part of this invention and'being well understood in the art do not require illustration or further description.

Coal is delivered by screw conveyor 6 into the bottom of the retort where it is picked up and moved upwardly by a vertical screw conveyor 8 operated by a gear 9 and a pinion III which may suitably be driven by the motor which operates screw 6. Screw conveyor 8 moves the coal upwardly through the retort into the active combustion zone and simultaneously agitates it. Disposed in wind box 4 is a partition H which divides it into an upper air chamber I! through which air is supplied to upper tuyeres 2 and 3, and a lower air chamber l3 through which air is supplied to the preoxidation tuyere l4. Mounted at the juncture of wind box 4 and the main air duct for pivotal movement about the end of partition II is a butterfly valve l5. By moving it approximately, as indicated by the broken line positions, Fig. '2,the air supplied through duct 5 may be apportioned in any desired manner between the upper and lower air chambers I2 and I3. Thus, all of the air may be passed to the upper tuyres .2 and 3, or all of it may be passed to preoxidation tuyere H. In ordinary practice, however, valve l5 will be adjusted to supply an amount of air to tuyere I4 which eifects preoxidation sufiicient to prevent coke tree formation, the remainder of the air-necessary for combustion being passed directly to the active combustion zone through upper air chamber l2 and tuyeres 2 and 3. A smoke back opening l6 may be provided as shown in Fig. 2 for the usual purpose of preventing back pressure at this point.

- wardly toward the active combustion zone. Air

for preoxidation enters tuyre H, at which point the temperature of the coalis sufllcient for initiation oi the preoxidation upon which the present invention is predicated. Under the combined action of the agitating action of conveyor I and V of the air introduced through tuyere .H, the coal is suiiiciently oxidized, by the timeit reachesthe plastic zone which lies immediately above the upper end of conveyor I, that coke trees do not form when the coal burns in the active combustion zone. Accordingly, the region between the plane of tuyere l4 and the top of conveyor 8 may be termed a preoxidation zone. Active combustion will initially occur in the region of tuyeres I and 3. In this manner the benefits of the invention, some of which have been described above, are attained. A further advantage of the use or a screw conveyor 8 for eflecting agitation is, as we have found, that it avoids segregation of lines in the coal such as may occur where such a conveyor is not used, which further assists in maintaining emcient operation.

Various modifications may, of course, be effected as will be understood from what has been said. Thus, preoxidation tuyere it might be located at a lower level, and other oxidizing agents than air may be used. We have found that benefits attend the use of a mixture of air and water vapor, and in a modified embodiment of I the stoker shown and described there is provided an injector or atomizer H for introducing a spray of water vapor into the air as it passes totuyere H. To avoid the induction of excessive amounts of water, injector ll may be actuated in any suitable manner to operate only when the blower is operating. As will be understood, from what has been said, the invention is applicable to alltypes of underfeed stolrers such, for example, as the clinkering, dry-ash or ash removal types.

According to the provisions of the patent statutes, we ha e explained the principle and mode of operatic of our invention and have illustrated and described what we now consider to represent its best embodiment. However, we desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

We claim:

1. An underfeed stoker comprising, in combination, a substantially vertical retort provided at its upper end with active combustion tuyeres forming an active combustion zone, a conduit connected to the lower end of said retort, means for passing coal through said conduit, a screw disposed in said retort with its upper end below said active combustion zone for moving coal upwardly from said conduit into said active combustion zone, a supplementary tuyere disposed below the upper end of said screw, the region between the upper end of the screw and said supplementary tuyere forming a preoxidation zone in which said screw mixes coal in a heated but non-plastic state substantially uniformly with air supplied from said'supplementary tuyere, a

tween said source and said active combustion and supplementary tuyeres, and proportioning means connected to said connections for passing a portion of the air to said active combustion zone and another portion to said supplementary tuyere. 1

2. An underfeed stoker comprising, in combination, a retort provided at its upper end with active combustion tuyeres forming an active combustion zone, a conduit connected to the lower end of said retort, means for passing coal through said conduit, a vertical screw disposed in said retort with its upper end below said active combustion zone for moving coal upwardly from said conduit into said zone, a supplementary tuyre in said retort below the upper end of said screw, the region between said supplementary tuyere and the upper end of said screw forming a preoxidation zone in which the coal in a heated but non-plastic and non-burning state is mixed by said screw substantially uniformly with air from said supplementary tuyre, a blower Ian, connections between said fan and said active combustion and preoxidation tuyeres, proportioncombustion zone with active combustion tuyres,

moving said coal with a vertical screw upwardly from said conduit to said active combustion zone, introducing into the upwardly moving coal, in a preoxidation zone below said active combustion zone and in which the upwardly moving coal is heated but not plastic and is not undergoing active combustion, air in an amount at least such as to oxidize the coal to an extent which represses subsequent coke tree formation, whereby under the action of said vertical screw the coal is agitated and substantial uniformity of particle size distribution is maintained throughout the upwardly rising coal and all of it is mixed substantially uniformly with said air and oxidized to said extent, and then moving the thus-oxidized coal into said active combustion zone and supplying a further amount of air to said active combustion tuyeres for active combustion of the coal.

4. In a method of operating an underieed stoker with coking coal in which the coal is passed through a conduit to the lower end of an upright retort provided in the active combustion zone' with tuyeres for supplying air for active combustion, and air is passed through said tuyeres for combustion, the improvement which consists in elevating the coal from said. conduit by a vertical screw in said retort, simultaneously supplying to the coal as it is carried upwardly by said screw about 15 to 35 per cent of the total air supplied for combustion, whereby as the uptained, and then moving the coal into said active combustion zone and supplying the balance oi the air for combustion to said tuyeres', and thereby avoiding coke tree formation in said active I combustion zones 5. An underi'eed stoker comprising, in combination, a substantially vertical retort provided at its upper end with active combustion tuyeres forming an active combustion zone, a conduit connected to the lower end of said retort, means for passing coal through said conduit, a vertical screw disposed in said retort with its upper end below said active combustion zone for moving coal upwardly from said conduit into said active combustion zone, a connection below the upper end of said screw for introducing into a preoxidation zone in which the coal is in a heated but non-plastic state and isnot undergoing active combustion at least about 15 per cent of the total air required for combustion oi the coal, and a connection 'to said tuyeres ior supplying the balance of the air for active combustion thereto.

THEODORE S. SPICER.

CALVERT C. 

